Phthalic anhydride is an important intermediate chemical in the chemical industry. One important use is in the production of alkyl phthalates such as di-isononyl or di-isodecyl phthalates which are used as plasticisers typically for polyvinyl chloride. These phthalates may be further hydrogenated to the corresponding di-cyclohexanoates. Phthalic anhydride has been produced on an industrial scale for many years, and has generally been produced by the vapour phase oxidation of ortho-xylene with an oxygen-containing gas such as air by passing a mixture of ortho-xylene and the oxygen-containing gas over an oxidation catalyst.
A typical plant for the production of phthalic anhydride comprises a raw material section, in which a hot mixture of the oxygen-containing gas and ortho-xylene vapour is prepared for feeding to the reactor which typically consists of reactor tubes containing catalyst. The reaction is exothermic and the temperature of the reactor tubes is controlled by a temperature control fluid, such as a molten salt, flowing around the tubes.
After the reaction, the crude phthalic anhydride that has been produced passes to a cooling stage where it is cooled, generally by a gas cooler, and is subsequently passed to optionally a liquid condenser and finally to a switch condenser. Finally the condensed phthalic anhydride is subjected to a purification or finishing step.
The efficiency of a phthalic anhydride plant is measured in terms of the number of grams of ortho-xylene that can be processed for each normal cubic meter of oxygen-containing gas or air that is fed to the raw material section (known as the loading). The greater the amount of ortho-xylene per unit of gas flow, the greater is the efficiency of the facility. Considerable attempts have been made over the years to increase the loading, and loadings above 80 gram/Nm3 of ortho-xylene in air have been reported.
One difficulty in the manufacture of phthalic anhydride is that, at the temperatures required for the reaction of air and ortho-xylene, the mixture becomes flammable and explosive at a loading above 44 gram of ortho-xylene per normal cubic meter of air. Accordingly, great care must be taken to avoid or reduce the likelihood of explosions. When an explosion occurs and the flame velocity exceeds the velocity of sound, this supersonic explosion is called a detonation. Otherwise, at subsonic flame velocities, it is called a deflagration. By the provision of an adequate number of escape ducts, such as chimneys, sealed off by rupture discs, at critical locations, the occurrence of a detonation is avoided, while the burning gas from a deflagration is relieved to a safe location. One or more rupture discs are conveniently located on the ortho-xylene vaporizer, at the reactor inlet and outlet, and on downstream equipment and the sections of the piping operating within the flammability limits. These rupture discs can be of any suitable design although reverse buckling or bending rod type are preferred. One of the areas in a phthalic anhydride facility that is prone to a deflagration is the raw material section where the ortho-xylene and the air are mixed. One of the reasons for a deflagration to occur is if there is incomplete vaporisation of the ortho-xylene or a concentration inhomogeneity in the vapour/air mixture at the time when it reaches the oxidation catalyst. Other reasons can be poor mixing of the heated ortho-xylene and the heated air, discharges from the build-up of static electricity, or the decomposition of peroxides formed from feed impurities like cumene or styrene. Also pyrophoric compounds may be formed and cause a deflagration.
The process therefore requires a mixture of oxygen-containing gas and ortho-xylene that is as homogenous as possible, operating at all points above the dew point of ortho-xylene. Typically liquid ortho-xylene is preheated to about 140° C. under elevated pressure, flow metered with mass flow meters, and forced into a spray nozzle configuration for injection into the heated oxygen-containing gas, which is typically air. The hot liquid ortho-xylene is thus sprayed as a fine mist into the hot oxygen-containing gas where the ortho-xylene is vaporised.
In a typical commercial process, the generation of a feed gas mixture has to date been performed as follows. Process air is sucked in from the surroundings through a filter by means of a blower, and compressed to a pressure level which allows the conveyance of the air stream through the phthalic anhydride plant. This process air stream is heated in a heat exchanger disposed downstream of the blower, and its flow is metered and controlled accurately. Parallel thereto, liquid ortho-xylene from a storage tank is brought to a preliminary pressure by means of a pump and passed through a basket type filter, an accurate flow meter and control device, and routed to a preheater before it is fed to an evaporator, vaporizer drum or spray drum. In the evaporator, the preheated ortho-xylene is injected in liquid form into the heated air stream, parallel to the air flow, by means of a nozzle system. The fine ortho-xylene droplets completely evaporate in the air stream, and a further smoothening of the radial concentration and temperature profiles in the gas stream is achieved by means of a homogenisation stage (a homogenizer section comprising e.g. a static mixer). This homogenised feed gas subsequently enters the reactor, typically a tubular reactor filled with catalyst, where a partial oxidation of ortho-xylene with atmospheric oxygen takes place to form phthalic anhydride. The oxygen is typically present in significant stoichiometric excess.
This process for the generation of feed gas has successfully been used, but with the successive introduction of higher ortho-xylene loads in the air stream (above 80 g of ortho-xylene per Nm3 air) the process has shown potential weaknesses with regard to the explosion safety of the raw material section of the plant. The lower explosion limit of a gaseous mixture of ortho-xylene and air is about 44 g of ortho-xylene per Nm3 of air. It has been found that the minimum energy required for igniting the mixture is greatly decreased with increasing ortho-xylene loading, and therefore the desire to increase the ortho-xylene loading increases the possibility of an explosion. However, to a great extent, the economics of the overall phthalic anhydride production process depends upon increasing the loading of ortho-xylene per Nm3 of oxygen-containing gas or air. It is therefore of basic importance that plants with a loading in the range of 80 g ortho-xylene/Nm3 air to 120 g ortho-xylene/Nm3 air must be operated safely.
U.S. Pat. No. 6,984,289 B2 relates to a process for the production of phthalic anhydride by the oxidation of ortho-xylene with air and with a loading of 80 g to 100 g of ortho-xylene per Nm3 of air. This higher loading is said to be made possible by complete evaporation followed by superheating of the ortho-xylene prior to admixture with air. DE 20 2005 012 725 U1 provides a system in which ortho-xylene is sprayed through nozzles into an air stream in which the flow cross-section of the air feed tube is reduced downstream of the spray nozzles, so that vapour velocity and turbulence are increased, thereby improving the mixing of the reaction components, and in this way the risk of explosion is reduced. DE 20 2005 012 725 U1 also provides a cone-shaped perforated screen at either side of the spray nozzles to divert the pressure wave from an explosion occurring in the evaporation section towards the rupture disks, thereby protecting the equipment upstream and downstream of these screens from damage by a shock wave. These screens assist also in homogenising the flow of air and the flow of the air/ortho-xylene mixture.
DE 10 2004 052 827 A1 is concerned with the preparation of a homogeneous mixture of ortho-xylene and air as a feed for the production of phthalic anhydride. DE 10 2004 052 827 A1 employs a series of arms or lances carrying spray nozzles to inject ortho-xylene into a stream of hot air. DE 10 2004 052 827 A1 further provides a sieve basket comprising concentric cones for directing the flow of air and homogenising the flow profile of the air entering the mixing chamber. In this way catalyst damage due to the carry over of unvaporised droplets of ortho-xylene onto the catalyst is reduced. According to DE 10 2004 052 827 A1 the ortho-xylene can be supplied to the nozzle from a supply line in which the ortho-xylene is held at a temperature above its flash point, typically in the range of 135-140° C.
A problem with the system of DE 10 2004 052 827 A1 is that erosion, particularly by cavitation of the ortho-xylene, can occur within the spray nozzles, which in turn leads to irregular spray patterns of ortho-xylene, perhaps leading to the formation of larger ortho-xylene droplets, and in turn leading to explosive conditions. It is believed that cavitation can be caused by the spontaneous formation of bubbles in the liquid within the spray nozzles which then collapse and in so doing they create shock waves in the liquid which erode and damage the nozzle material surfaces.
US 2003/0013931 also discloses a process and apparatus for the production of a homogeneous mixture of ortho-xylene and air in the production of phthalic anhydride, wherein the ortho-xylene is atomised by means of six axial hollow cone swirl nozzles. US 2003/0013931 is concerned with avoiding upsets in the homogeneity of the o-xylene vapor/air mixture produced by fluctuating operating parameters, The document is not concerned with erosion of the nozzles or the resulting damage to nozzle material surfaces.
Spray nozzles are offered in a wide variety of construction materials, including various polymers. In the chemical process industry, the typical need to resist contact with organic liquids reduces the choice in construction materials to the specialised polymers and metals. Metals are typically preferred because of their typically higher mechanical strength. We have however found that many of the metals of which spray nozzles are made show weaknesses in the production of the mixture of ortho-xylene and oxygen-containing gas, as they are insufficiently resistant to erosion, in particular by cavitation. Many metals are also subject to corrosion under the hot and humid conditions occurring in the vaporiser. Both of these weaknesses may lead to deformation over time of the nozzle surfaces in contact with the liquid ortho-xylene, such that the sprayed mist of ortho-xylene becomes less homogeneous. The droplet size distribution becomes broader and the larger droplets may take more time to evaporate. The droplet density of the mist also becomes less uniform and coalescence may occur in areas of higher droplet density. The lower homogeneity of the mist may therefore lead to an increased risk for deflagrations over time.
A further problem associated with the spray nozzle system is to provide an effective seal where the spray nozzles are connected to the rest of the nozzle system, to prevent leakage of ortho-xylene and the development of larger liquid droplets which, as mentioned earlier, can lead to explosive conditions. It has been suggested to use polytetrafluoroethylene tape to create a seal between the nozzle and the rest of the nozzle system, but this has the disadvantage that the polymeric tape is not electrically conductive and its use can lead to electrostatic build up, increasing the risk of explosion. Furthermore the polytetrafluoroethylene tape may be torn apart particles can be created inside the nozzle which may destroy the flow pattern, leading to the formation of larger droplets of ortho-xylene and the risk of explosions.
It is important that a homogenous mixture of ortho-xylene and oxygen-containing gas is formed for feeding to the reactor and this may be accomplished by enhancing the rate of ortho-xylene vaporisation. Furthermore it is important that the ortho-xylene does not coalesce or condense and form liquid deposits within the raw material section of the plant, to reduce the risk of explosion when liquid deposits are formed. We have found that this may be accomplished by employing a particular nozzle system and a particular set of conditions within the nozzle to spray the ortho-xylene into the hot oxygen-containing gas. In addition, we provide a particular sealing system to prevent the leakage of liquid ortho-xylene. The nozzle system of this invention, including the sealing system, is particularly useful when used in combination with a specially designed oxygen-containing gas feed system and a particular design of oxygen-containing gas and ortho-xylene mixing system.